Polyvinyl chloride resins, polystyrene resins, and the like are well known as materials widely used for shrink films, however, the by-products of the discarded resins are detrimental to humans and the environment. Therefore, shrink films made from polyolefins are under development today. Shrink films produced from conventional polyolefin resins have inferior mechanical properties, thermal shrinkage at low temperatures, and other such characteristics as compared to those made from vinyl chloride resins. Particularly when these films are used as shrink labels for PET beverage bottles, the film is usually shrunken along with the PET bottle in a shrink tunnel using steam or the like, and therefore a shrink film that exhibits a high shrinkage factor at lower temperatures is demanded. Furthermore, when PET bottles are recycled, their labels are pulverized along with the PET resin itself, with the label resin usually being separated from the PET resin of the bottle by a liquid specific gravity separation process that uses the difference in buoyancy in water between these materials. However, a polystyrene resin, for instance, has a specific gravity of about 1.03 to 1.06, and sinks in water along with a PET-based resin, whose specific gravity is 1.3 to 1.5. Therefore, labels made from a resin having a specific gravity of 1 or higher are difficult to separate from a PET resin by the above method. Accordingly, there is a demand for a shrink film that has a specific gravity of less than 1, is made from a polyolefin, and shrinks at low temperatures.
For example, Japanese Laid-Open Patent Application 2001-109429, discloses a “Heatshrinkable Polypropylene-based Film” where the comonomer content of a polypropylene-based film is 27%. This film has excellent transparency and heat-shrinkability at low temperatures, however, it does not necessarily have the rigidity and heat resistance required for a film. Also, the natural shrinkage factor ends up being large when an attempt is made to raise the heat shrinkage factor of a shrink film made from a polyolefin such as polypropylene. Consequently, there is a demand for a shrink film that strikes a good balance between heat shrinkage factor and natural shrinkage factor.
It is an object of the present invention to provide a shrink (label) film that is based on a polypropylene resin with excellent rigidity and transparency, that can be stretched at low temperatures, namely at 80° C. or lower, that has a shrinkage factor at 80° C. of at least 30%, and preferably at least 35%, and a shrinkage factor at 90° C. of at least 40%, and preferably at least 45%, and whose appearance is good enough for practical application.